The aim of the proposed research is to examine in detail the genetic consequences of the repair of damaged DNA in irradiated and unirradiated cells of the eukaryote Saccharomyces cerevisiae. The chief method by which this will be achieved entails the combined use of radiation sensitive strains, presumed to be deficient in repair, and alleles of the structural gene for iso-l-cytochrome c which have been characterised by analysis of revertant proteins. We have already shown that there is a single error-prone or mutagenic repair pathway for UV damage in yeast involving the action of at least seven genes (rad6, rad8, rad9, rad18, rev1, rev2, and rev3). The proposed work will examine the properties of the genes active in this pathway further from three points of view. First, their involvement in ionizing radiation mutagenesis will be examined. Second, the allele specific influence of several of the genes will be investigated using the reversion of a variety of well defined cycl alleles. Third, temperature conditional mutants will be used to examine the physiology of error-prone repair. BIBLIOGRAPHIC REFERENCES: C.W. Lawrence, J.W. Stewart, F. Sherman and R. Christensen, Influence of repair on the specificity of ultraviolet induced reversion of an ochre allele of the structural gene for iso-l-cytochrome c. In Molecular Mechanisms for the Repair of DNA, edited by P.C. Hanawalt and R.B. Setlow, Plenum Press, 1975, P.397. C.W. Lawrence and R. Christensen, UV mutagenesis in radiation sensitive strains of yeast. Biophysical Journal 15: No. 2, part 2, 1975 (Abstract).